The present invention relates to a hermetic scroll compressor using helium gas as a working gas and for use in a helium compressor for a cryopump device in an extreme high vacuum field.
A conventional hermetic scroll compressor in which cooling is effected by oil, that is, an oil injection type hermetic scroll compressor is disclosed in, for example, JP-A-61-187584 specification. In such conventional hermetic scroll compressor, an oil separator is provided midway in a discharge pipe, and oil separated from gas at the oil separator is injected into a compressor section, and oil within a hermetic container is also injected into the compressor section. Further, an example of an oil injection type scroll compressor for a helium liquefied refrigerating apparatus is disclosed in JP-A-3-271583 specification. In the above-described scroll compressors, a number of turns of scroll wrap has been set to be as comparatively large as 2.7 to around 4.
Recently, when a compressor is used for helium application, need for an operation within a hitherto unknown, exceedingly low operating pressure ratio range (operating pressure ratio is discharge pressure/suction pressure), for example, a range of around 1.5 to around 2 has been increased. In conventional compressors for cryopump apparatus in the above-described oil injection type conventional compressors, however, an operating pressure ratio is around 2.5 to around 7, and therefore, any normal operation cannot be secured in such exceedingly low operating pressure ratio range. In other words, in the case of low operating pressure ratio of around 1.5 to around 2 as described above, a problem arises that excessive compression power loss of the compressor generally becomes high and efficiency reduction becomes remarkable.
Further, in the conventional compressors, since an amount of displacement of an orbiting scroll in an axial direction is increased by an increase in gas pressure in a compression chamber, the orbiting motion may become unstable, causing galling or the like due to contact between wraps. Since when helium gas is used as working gas, it has a higher ratio of specific heat than other refrigerant gas (for example, freon gas for refrigeration and air conditioning), an internal compression power also becomes greater, and discharge gas temperature becomes higher, and therefore, it is necessary to strengthen cooling of the helium gas.
It is an object of the present invention to provide a scroll compressor capable of performing a stable operation without lowering the efficiency even within an exceedingly low operating pressure ratio range.
In order to attain the above-described object, a scroll compressor using helium gas as working gas according to the present invention, comprises a fixed scroll and an orbiting scroll, engaged with each other, each having a scroll tooth profile shape of a set volume ratio of a scroll wrap portion of 1.8 to 2.3, said set volume ratio being defined by the following expression;                     Vr        =                                            2              ⁢                              λ                1                                      -                          4              ⁢              π                        +            α                                              2              ⁢                              λ                s                                      +                          2              ⁢              π                        +            α                                              (        1        )            
where
Vr: set volume ratio of a scroll wrap portion,
xcex1: an angle at a wrap winding end portion (Involute angle),
xcexs: an angle at a wrap winding start portion (Involute angle),
xcfx80: circle ratio,
xcex1: ratio of an orbiting radius xcex5th to a base circle radius a of the scroll wrap (=xcex5th/a).
The set volume ratio of a scroll wrap portion=volume of maximum closed space formed by scroll wraps/volume of minimum closed space formed by scroll wraps.
FIG. 17 shows variations in overall adiabatic efficiency with respect to an operating pressure ratio when the set volume ratio of a scroll wrap portion Vr is changed. When the set volume ratio of a scroll wrap portion Vr is 1.8 to 2.3, the overall adiabatic efficiency of the present invention increases over ten percent to over twenty percent in terms of the operating pressure ratio than a conventional compressor (of which set volume ration Vr is 2.7, for example). Particularly, the scroll compressor using helium gas is frequently operated at an extremely low operating pressure ratio, but according to the above-described structure, the overall adiabatic efficiency increases at the exceedingly low operating pressure ratio and a high-performance scroll compressor can be obtained.
The scroll wrap portion is formed of a casting material, and at least one of a scroll wrap winding start portion and a scroll wrap winding end portion can be provided with a step.
Further, it is preferable to set the set volume ratio Vr to 2.1. A reason why the set volume ratio Vr is set to 2.1 is that, as can be appreciated from FIG. 17, there is an effect of improving the performance at the low operating pressure ratio range while the performance reduction at the high operating pressure ratio range (around 5) is comparatively low.
Also, when tip ends of the scroll wrap portion are rounded with a radius larger than a half of an average thickness of the scroll wrap portion, the strength of the scroll wrap can be improved.
Also, the fixed scroll is fixed to a frame provided on a back surface side of the orbiting scroll. A back pressure chamber is formed by the frame and a back surface of an end plate of the orbiting scroll. An oil filler port, through which oil for cooling the helium gas is injected into a compression chamber formed by the fixed scroll and the orbiting scroll, is provided on an end plate of the fixed scroll. A thin hole, through which a part of the helium gas halfway of compression stroke within the compression chamber is introduced into the back pressure chamber, is formed on the end plate of the orbiting scroll. A ratio Pb/Ps of the pressure Pb within the back pressure chamber with respect to a suction pressure Ps is set to around 1.5.
Also, in the present invention, a lower limit of the operating pressure ratio (Pd/Ps)min and a set intermediate pressure ratio (Pb/Ps)m satisfy a relationship of (Pd/Ps)minxe2x89xa1(Pb/Ps)m+0.2.
Further, a compressor of the present invention is characterized in that it is applied to a helium refrigerating apparatus and it is operated at a condition in which an operating pressure ratio of is around 1.5 to around 5 and an upper limit of the suction pressure is set on a lower limit of the operating pressure ratio.
In addition, with the aim of enhancing the performance at a low operating pressure ratio range of around 1.5 to around 2, the present invention is characterized by having a fixed scroll and an orbiting scroll, engaged with each other, each having a scroll tooth profile shape of a set volume ratio Vr of a scroll wrap portion of 1.8 to 2.0.